1. Field of the Invention
The present invention relates to fluorescent-lamp-driving device that is applicable to a backlight device driving a plurality of hot cathode fluorescent lamps (HCFL), and a liquid crystal display apparatus using the same.
2. Description of Related Art
The backlight device has recently been used in a liquid crystal television or a liquid crystal monitor using a large-scaled liquid crystal display panel. As light source of the backlight device, fluorescent lamps such as a plurality of cold cathode fluorescent lamps (CCFL), a plurality of external electrode fluorescent lamps (EEFL), or LED elements arranged like grid array are often used. As the fluorescent lamps, a plurality of HCFLs each having heaters may have also been used other than CCFL and EEFL.
HCFL has the same structure as that of fluorescent light tube used in a widely distributed lighting apparatus or the like for consumer electrical appliances and has excellent color reproductivity, good luminance efficiency and low applied voltage so that it has excellent characteristics as compared with those of CCFL. HCFL, however, may be necessary for heaters of both the end of the lamp so that it has a complex circuit, which results in high costs. This is because HCFL has not often used in the liquid crystal display. The fluorescent lamp generally contains fluorescent-lamp-driving device because the fluorescent lamp is driven by an alternating current. The fluorescent-lamp-driving device has often a function such that its output current is controlled so as to become a constant and keep its luminance constant even if input direct current voltage varies or impedance of the fluorescent lamp varies.
FIG. 1 shows a configuration of a backlight device 1 related to a related art using n pieces of HCFLs. The backlight device 1 shown in FIG. 1 contains a driving control circuit 2, a driving circuit 3 of high voltage side, a driving circuit 4 of low voltage side, a transformer 5 for inverter, two heater transformers HT1 and HT2 of high voltage side, n pieces of heater transformers LTi (i=1 to n) and n pieces of balance transformers BTi (i=1 to n).
The driving control circuit 2 is connected with a primary winding w1 of the transformer 5 for the inverter. A secondary winding w3 of the transformer 5 for the inverter is connected with heaters Hh1, Hh3, . . . , Hhn−1 of high voltage side of odd-numbered fluorescent lamps L1, L3, . . . , Ln−1 in parallel. A secondary winding w2 of the transformer 5 for the inverter is connected with heaters Hh2, Hh4, . . . , Hhn of high voltage side of even-numbered fluorescent lamps L2, L4, . . . , Ln in parallel. The transformer 5 for inverter supplies alternate current voltage having a predetermined operating frequency that may be necessary for discharging.
The driving circuit 3 of high voltage side drives the heaters Hh1, . . . , Hhn of high voltage side via the heater transformers TH1 and TH2 of high voltage side with the fluorescent lamps being divided into two groups of the odd-numbered fluorescent lamps L1, L3, . . . , Ln−1, and the even-numbered fluorescent lamps L2, L4, . . . , Ln in order to equalize luminance of n pieces of fluorescent lamps L1, . . . , Ln. Thus, the driving circuit 3 of high voltage side is connected with the heater transformers HT1 and HT2 of high voltage side. The heater transformer TH1 of high voltage side is connected with the heaters Hh1, Hh3, . . . , Hhn−1 of high voltage side of odd-numbered fluorescent lamps L1, L3, . . . , Ln−1 in parallel. The heater transformer TH1 of high voltage side supplies the heaters Hh1, Hh3, . . . , Hhn−1 of high voltage side of odd-numbered fluorescent lamps L1, L3, . . . , Ln−1 with heater power.
The heater transformer TH2 of high voltage side is connected with the heaters Hh2, Hh4, . . . , Hhn of high voltage side of even-numbered fluorescent lamps L2, L4, . . . , Ln in parallel. The heater transformer TH2 of high voltage side supplies the heaters Hh2, Hh4, . . . , Hhn of high voltage side of even-numbered fluorescent lamps L2, L4, . . . , Ln with heater power.
The driving circuit 4 of low voltage side is connected with a primary side of each of n pieces of the heater transformers LTi (i=1 to n) and supplies the heaters Hl1, . . . , Hln of low voltage side of n pieces of fluorescent lamps L1, . . . , Ln with heater power. A secondary side of the heater transformer LT1 of low voltage side is connected with the heater Hl1 of low voltage side of the fluorescent lamp L1. A secondary side of the heater transformer LT2 of low voltage side is connected with the heater Hl2 of low voltage side of the fluorescent lamp L2. Similarly, a secondary side of the heater transformer LTn of low voltage side is connected with the heater Hln of low voltage side of the fluorescent lamp Ln.
The balance transformer BT1 for detection of lamp current is connected with a terminal of the heater Hl1 of low voltage side of the fluorescent lamp L1. The balance transformer BT2 therefor is connected with a terminal of the heater Hl2 of low voltage side of the fluorescent lamp L2. Similarly, the balance transformer BTn therefor is connected with a terminal of the heater Hln of low voltage side of the fluorescent lamp Ln.
A terminal of a secondary side of the balance transformer BT1 is connected with a terminal of a secondary side of the other balance transformer BT2 in series. The other terminal of the secondary side of the balance transformer BT2 is connected with a terminal of a secondary side of the other balance transformer BT3 in series. Similarly, the secondary side of the balance transformer BTn is connected with a terminal of a terminal of the heater Hln of low voltage side of the fluorescent lamp Ln. The n pieces of balance transformers BT1, . . . , BTn detects lamp load current that runs in each of n pieces of fluorescent lamp L1, . . . , Ln serially and outputs a lamp current detection signal S4 indicating a sum total of the lamp load current by the n pieces of fluorescent lamp L1, . . . , Ln. The backlight device 1 having such a configuration can control luminance of the n pieces of fluorescent lamp L1, . . . , Ln so as to become a constant based on the lamp current detection signal S4, so that it is possible to construct a liquid crystal display apparatus having characteristics such as an excellent color reproductivity, good luminance efficiency and low applied voltage which are excellent ones as compared with those of CCFL or the like.
Japanese Patent Application Publication No. S63-190297 has disclosed on page 3 and FIG. 1 a discharge-lamp-lighting device. The discharge-lamp-lighting device contains a timer circuit, an inverter circuit, a current-limiting element and pre-heating circuit. The timer circuit is connected with an electric power supply and a control signal is output after a set period of time has been elapsed from a point of power-on time. The inverter circuit is connected with the timer circuit and the discharge lamp is connected with the inverter circuit through the current-limiting element. The inverter circuit includes at least one of pre-heating winding which supplies the discharge lamp with low voltage such that the discharge does not start based on the control signal during the set period of time from a point of power-on time.
After the set period of time has been elapsed from the point of power-on time, the inverter circuit supplies the discharge lamp with high voltage such that the discharge can start based on the control signal. On the assumption of this, the pre-heating circuit includes a voltage detection device and is connected with the pre-heating winding of the inverter circuit so that it detects that output voltage of the pre-heating winding is low and pre-heats electrodes of the discharge lamp. The apparatus having such a configuration can combine the timer circuit (lighting circuit) and the pre-heating circuit as one piece of power circuit, which enables the lighting device to be downsized and to be reduced in weight.
Japanese Patent Application Publication No. H06-045079 has disclosed on page 2 and FIG. 1 a luminescent-lamp-lighting device. The luminescent-lamp-lighting device contains a transistor inverter having a resonance circuit and an output transformer, and a filament pre-heating member. A switching transistor is connected with a direct current power supply and by switch operation of this transistor, an alternate current voltage generated in the resonance circuit is output to the output transformer. The output transformer is connected with a luminescent lamp. On the assumption of this, the filament pre-heating member limits operating frequency of the switching transistor to an operating frequency such that it is difficult to start the luminescent lamp because the filament is pre-heated until a set period of time has been elapsed from a point of direct-power-on time. After the set period of time has been elapsed from the point of direct-power-on time, the filament pre-heating member changes the operating frequency of the switching transistor to an operating frequency such that the luminescent lamp can start. The device having such a configuration enables blackening due to cold start of the luminescent lamp, shortening of lift time of the luminescent lamp or the like to be prevented.
Japanese Patent Application Publication No. 2001-338790 has disclosed on page 3 and FIG. 1 a discharge-lamp-lighting device and an illumination apparatus. The discharge-lamp-lighting device contains a direct-current power supply, first and second switching members, resonance inductance, resonance capacitance, a drive resonance circuit, temperature-sensitive resistor, and a drive signal generation circuit of feed-back type. The first and second switching members are connected to each other in series and are connected with the drive signal generation circuit. The resonance inductance is connected with the drive signal generation circuit. The discharge lamp is connected with the resonance inductance. First resonance capacitance is connected with electrodes of the discharge lamp and the electrodes of the discharge lamp are connected with the direct-current power supply through second resonance capacitance.
According to this discharge-lamp-lighting device, the discharge lamp is driven based on high frequency alternating current generated by the alternative switching operations by the first and second switching. The drive resonance circuit resonates based on feed-backed voltage generated at the resonance inductance that feeds back the current flown through the discharge lamp. On the assumption of this, the temperature-sensitive resistor is connected with the drive resonance circuit and resonance frequency of the drive resonance circuit varies in succession at a point of power-on time. The drive signal generation circuit controls the first and second switching members so as to switch them on alternatively based on the resonance voltage of the drive resonance circuit.
The discharge-lamp-lighting device having such a configuration can start the operation of discharge lamp after pre-heating the filament electrodes at a point of power-on time, thereby improving a feature of switching the discharge lamp on and off. At the same time, the temperature-sensitive resistor connected with the drive resonance circuit operates at a relative low voltage so that reliability thereof can be improved.